A well known form of reverse circulation drilling uses a percussive type air operated hammer where high pressure air is used to operate the hammer and exhaust air is used to clear the cuttings as described above.
A transmission sleeve is described in an earlier Australian Patent No. 638571. That specification describes a sacrificial transmission sleeve that is used to form a seal and also to direct exhaust air to the cutting face to assist in the clearance of chips. The invention described in the specification was an important development in reverse circulation hammers and is now commonly used in the drilling industry. Another relevant patent is Australian Patent No. 656724.
The present invention relates to improvements to the transmission sleeve. These improvements relate both to the air direction provided by the transmission sleeve and to the design of the drill bit. The sealing requirement of the improved transmission sleeve is preferred in relation to air operated reverse circulation hammers. The transmission sleeve forms an effective seal with the hole being drilled where the passage of air in an upward direction past the transmission sleeve is restricted. It will be obvious to a person skilled in the art that some form of clearance between the transmission sleeve and the hole of the wall is required. However, the clearance will vary depending on ground conditions. It is normal practice to maintain a clearance between the gauge diameter of the drill bit and the transmission sleeve between one to three millimetres. However, other clearances, either larger or smaller, may still be suitable.